core: refactor timers

[saghul]

• Don't use JS objects for timer handles
• Use a hash table in C rather an a Map in JS
• Allocate memory without using the JS allocator (1)

1. This is important because the libuv loop outlives the JS runtime.
   Since handle closing is not sync and the callback where we need to
   free the structures will be called later it's not safe to use it.
   This is a small step in the direction of not using the JS allocator
   for structures that embed libuv handles.